Thought I'd share a good experience. I bought a secondhand F3 which had just the tiniest speck of dist on the sensor. An initial attempt to dislodge it simply resulted in more dust appearing on it which really stuffed it's use. For a while I thought I was in for a very expensive sensor replacement. Until I happened across the Broadcast Service Centre in Wokingham http://www.bsc-online.co.uk/
They actually created a dust-free work area specifically to dismantle and clean my sensor assemble and did a great job, so I'm back to using my F3 again. The cost was around £350+VAT, which is pretty small beer in the scale of things, so I thought I'd share in case other F3 owners need a similar job doing.